LLVM CI Best Practices — LLVM 22.1.3 documentation

# LLVM CI Best Practices

## Overview

This document contains a list of guidelines and best practices to use when working on LLVM’s CI systems. These are intended to keep our actions reliable, consistent, and secure.

## GitHub Actions Best Practices

This section contains information on best practices/guidelines when working on LLVM’s GitHub actions workflows.

### Disabling Jobs In Forks

There are many LLVM forks that exist, and we currently default to preventing actions from running outside of the LLVM organization to prevent them from running in forks. We default to this as actions running in forks are usually not desired and only run by accident. In addition, many of our workflows assume that they are operating within the main LLVM repository and break otherwise.

Adhering to this best practice looks like adding the following to each of the jobs specified within a workflow:

    jobs:
      <job name>:
        if: github.repository_owner == 'llvm'

We choose to use `github.repository_owner` rather than `github.repository` to enable these workflows to run in forks inside the LLVM organization, such as the ClangIR fork.

There are some exceptions to this rule where `github.repository` might be used when it makes sense to limit a workflow to only running in the main monorepo repository. These include things like the issue subscriber and release tasks, which should not run anywhere else.

### Hash Pinning Dependencies

GitHub Actions allows the use of actions from other repositories as steps in jobs. We take advantage of various actions for a variety of different tasks, but especially tasks like checking out the repository, and downloading/uploading build caches. These actions are typically versioned with just a release, which looks like the following:

    steps:
      - name: Checkout LLVM
        uses: actions/checkout@v4

However, it is best practice to specify an exact commit SHA from which to pull the action, noting the version in a comment:

We plan on revisiting this recommendation once GitHub’s immutable actions have been rolled out as GA.

    steps:
      - name: Checkout LLVM
        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2

This is beneficial for two reasons: reliability and security. Specifying an exact SHA rather than just a major version ensures we end up running the same action originally specified when the workflow was authored and/or updated, and that no breaking changes sneak in from new versions of a workflow being released. However, this effect could also be achieved by specifying an exact dot release. The biggest reason to prefer hash pinned dependencies is security. Release assets on GitHub are mutable, allowing an attacker to change the code within a specific version of an action after the fact, potentially stealing sensitive tokens and credentials. Hash pinning the dependencies prevents this as the hash would change with the code.

### Using Versioned Runner Images

GitHub actions allows the use of either specifically versioned runner images (e.g., `ubuntu-22.04`), or just the latest runner image (e.g., `ubuntu-latest`). It is best practice to use explicitly versioned runner images. This prevents breakages when GitHub rolls the latest runner image to a new version with potentially breaking changes, instead allowing us to explicitly opt-in to using the new image when we have done sufficient testing to ensure that our existing workflows work as expected in the new environment.

### Top Level Read Permissions

The top of every workflow should specify that the job only has read permissions:

    permissions:
      contents: read

If specific jobs within the workflow need additional permissions, those permissions should be added within the specific job. This practice locks down all permissions by default and only enables them when needed, better enforcing the principle of least privilege.

### Ensuring Workflows Run on the Correct Events

GitHub allows workflows to run on a multitude of events, and it is important to configure a workflow such that it triggers on the correct events. There are two main best practices around events that trigger workflows:

1. Workflows that are designed to run on pull requests should not be restricted by target branch. Restricting the target branch unnecessarily will prevent any stacked PRs from being tested. `pull_request` events should not contain a branches key.

2. Workflows that are designed to also trigger on push events (e.g., for testing on `main` or one of the release branches) need to be restricted by branch. While pushes to a fork will not trigger a workflow run due to the `push` event if the workflow already has its jobs disabled in forks (described above), stacked PRs will end up running jobs twice if the `push` event does not have any branch restrictions. `push` events should have their branches restricted at the very least to `main` and the release branches as follows:

    push:
      branches:
        - main
        - release/*

### Make Workflows Run on Updates to the Workflow Definition

Whenever possible, workflows should also run whenever the workflow definition is updated. This enables easily testing the workflow whenever modifying it. For example, if we have a workflow with a definition in `.github/workflows/foo.yaml`, we should have at least the following event within the workflow:

    pull_request:
      paths:
       - .github/workflows/foo.yaml

Note that it is not always possible to enable this (e.g., issues that use a `workflow_run` trigger). But when possible, this makes testing the workflow much simpler.

## Container Best Practices

This section contains best practices/guidelines when working with containers for LLVM infrastructure.

### Using Fully Qualified Container Names

When referencing container images from a registry, such as in GitHub Actions workflows, or in `Dockerfile` files used for building images, prefer fully qualified names (i.e., including the registry domain) over just the image. For example, prefer `docker.io/ubuntu:24.04` over `ubuntu:24.04`. This ensures portability across systems where a different default registry might be specified and also prevents attackers from changing the default registry to pull in a malicious image instead of the intended one.